During adsorption, the residual forces of the surface always decrease. As a result, there is a decrease in surface energy, which emits as a heat.
Of adsorption is always negative. The freedom of movement of adsorbate (gas) molecule becomes restricted when the adsorbent adsorbs the adsorbate(gas). As a result, the entropy decreases ( is negative). For the process to be spontaneous should be negative. .Since is negative, has to be negative, so that is negative. Hence adsorption is always exothermic.